The invention relates to magnetic particles and a method for making the same. More particularly, it relates to durable magnetic particles suitable for use in electromagnetic connecting devices such as electromagnetic clutches.
An electromagnetic clutch is designed to transmit a torque through a medium in the form of magnetic particles. Frequent use of the clutch or the use of the clutch under a high load would cause magnetic particles to be broken into finer particles or overheat magnetic particles so that the particles are oxidized at their surface to deteriorate their magnetic properties. This is particularly true when the magnetic particles are of pure iron. As a result, the torque transmitted by the clutch is reduced.
One solution is to improve the durability of magnetic particles. It is proposed to form magnetic particles from iron base material having Al, Si, Cr, B or Nb added thereto as disclosed in Japanese Patent Publication No. 56-31451. Another attempt is to plate Cr or A1 on the surface of pure iron particles. These attempts are unsuccessful in satisfactorily improving durability.
It is also proposed to use amorphous magnetic powder in order to obtain magnetic particles of high hardness as disclosed in Japanese Patent Application Kokai Nos. 59-159435 and 61-175329. Currently available amorphous magnetic powders are as hard as a Vickers hardness of higher than about 700, but unreliable under the influence of heat. They are not available as spherical particles capable of efficient torque transmission.